Optical storage systems typically use a radiation beam generated and processed in an optical head to record data on and/or retrieve data from an optical storage medium. Many of these systems utilize differential detection in order to detect small reflected signal fluctuations in the presence of other signal components and/or system noise. An example is a conventional magneto-optic (MO) system, in which data is stored on an MO medium in the form of marks having a distinct magnetic orientation. MO systems generally utilize Kerr rotation of a return beam reflected from the MO medium to distinguish marked and unmarked areas. The Kerr rotation produces relatively small variations in the return beam and is therefore difficult to detect without differential detection. Differential detection channels are typically provided in the MO system by separating the return beam into two orthogonal polarization components using a polarization beam splitter. The components are incident on separate detectors, and the resulting detected signals are applied to inputs of a differential amplifier which generates a differential MO data signal representative of the stored data.
In systems with differential detection channels, it is usually important to maximize the common-mode noise rejection in order to ensure optimal performance. Significant degradations in output data signal carrier-to-noise ratio (CNR) may result if, for example, one or more of the elements in the differential channels do not provide substantially equivalent gain and/or phase variations. Prior art techniques addressing this problem have utilized, for example, strict optical head alignment and performance tolerances, or variable gain components in one or more of the differential detection channels. U.S. Pat. No. 4,691,308 discloses an MO system with differential detection channels and a variable gain in one channel. The variable gain is adjusted in response to an error signal corresponding to amplitude differences between the detected signals. The variable gain adjustment attempts to reduce the amplitude difference between the detected signals such that common-mode rejection in the differential amplifier is improved. However, this one-channel variable gain system is susceptible to a number of problems, including long-term drift in signal levels, variable phase shifts as a function of signal level, and poor recovery from non-ideal conditions such as out-of-focus or media defects. Other problems with one-channel variable gain systems include the inability to adequately compensate for spurious output signal modulation resulting from, for example, media birefringence.
Japanese Patent Publication No. 4-298836 entitled "Magneto-optical Recording and Reproducing Device" appears to disclose an MO detection system which uses a pair of level control circuits controlled in accordance with "double refractivity information." However, this system does not appear to improve common-mode rejection in differential detection. Furthermore, it apparently utilizes a common control signal for both level control circuits and thus fails to solve the long-term drift, output signal modulation and other problems inherent in the one-channel variable gain system of U.S. Pat. No. 4,691,308.
Optical systems with differential detection channels can also be used to generate a density-type data signal from a write-once (WO) medium by summing the two detected signals. As the term is used herein, WO media are intended to include read-only media such as compact disks (CDs) which are usually generated from a master recording. A system which generates a WO data signal is often susceptible to the effects of a number of different types of system noise. For example, a laser diode or other optical source used to read recorded data may exhibit mode-hopping or other instabilities which cause variations in the power level of the read beam. Such instabilities may be generally referred to as optical source noise or relative intensity noise (RIN). RIN represents a type of common-mode noise, that is, a noise component which is common to differential detection channels in the optical head. As noted above, common-mode noise can be substantially eliminated in generating an MO data signal because the detected signals are subtracted. However, common-mode noise remains in a WO data signal in which the detected signals are summed.
An exemplary technique which uses subtraction of a front facet monitor signal to limit the effects of RIN and other types of common-mode system noise on a WO data signal is described in U.S. Pat. No. 5,363,363 entitled "Apparatus and Method For Laser Noise Cancellation in an Optical Storage System Using a Front Facet Monitor Signal," which is assigned to the assignee of the present invention and incorporated by reference herein. One embodiment of the technique involves subtracting a front facet monitor (FFM) signal representative of the optical source power level from the WO data signal in a differential amplifier. The resulting noise reduction generally depends upon proper gain and phase matching of the data and FFM signal channels. Commonly assigned U.S. patent application Ser. No. 309,837 filed Sep. 21, 1994 by Dohmeier et al, entitled "Apparatus and Method for Controllable Gain and Phase Matching in an Optical Data Storage System with Source Noise Subtraction", now U.S. Pat. No. 5,491,682, discloses the use of a variable gain servo loop to match the amplitude and phase of the data and FFM signals prior to subtraction. The gain of either the data or FFM signal path is varied in accordance with, for example, an error signal corresponding to low-frequency amplitude differences between the data and FFM signals.
Although the above-noted exemplary source noise subtraction techniques provide considerable improvement in WO data signal quality, there remains a need for a universal optical system which can provide both improved differential detection for MO signals as well as source noise subtraction for WO signals in a simplified implementation which is suitable for digital control and avoids the spurious modulation, long-term drift and other problems of prior art systems.